While initiating a polymerisation reaction in an autoclave used to manufacture PVC emulsion in a chemical plant, a leak of vinyl chloride monomer (VCM) occurred on a gasket of a 1 m diameter flange. The procedure to inhibit the reaction was initiated. As the autoclave contained 7.5 t of VCM, the leak continued under the supervision of the in-house firefighters for 2 hours in the aqueous phase and for 3.5 hours in the gaseous phase. The production shop and the neighbouring workshop were shut down; 500 kg of vinyl chloride was released, primarily into the atmosphere. A VCM sensor in the workshop where the leak occurred had detected up to 80% LEL, and measurements performed in the site’s environment were negative.
Incorrect tightening of the flange bolts, which had been refitted a few weeks earlier after a [pressurised vessel] requalification operation, was identified as the cause of the leak. The composite gasket, made up of several overlapping elements, had slipped and was destroyed at a point under the device’s pressure (8 bar). The incorrect tightening of the bolts was associated with the use of an undersized pneumatic impact wrench (pneumatic screwdriver) by a subcontractor which had performed similar multiple tightening operations throughout the factory. A pressure decrease in the compressed air system, following the workshop’s shutdown, accentuated this incorrect tightening. This most likely resulted in a probable loss of pneumatic tool performance over time.
The operator implemented the following measures following the accident:
- double-checking of the tightness of the flanges on similar devices that have recently undergone requalification operations,
- external companies are reminded of the tightening protocols, as well as the types of pneumatic impact wrenches to be used (minimum tightening torque to be reached, even if the machine is heavier).
The operator also implemented procedures for the periodic inspection of the performance characteristics of pneumatic impact wrenches, verification of tightening and associated audits.
